In a process for producing a lithographic printing plate, there has been desired a direct plate making process in which an image is directly formed based on digital informations stored in a computer, a facsimile machine and the like, without a positive or negative film. As one of the direct plate making processes, it has been proposed to form an image by ink-jet printing. The ink-jet printing can output the information relatively high speed and is also a simple system in comparison with the other systems which need a complicated optical system. The simple system reduces maintenance fee, thus making cost reduction down of the plate making process.
Japanese Kokai Publication 2-286335 proposes a direct plate making process using the ink-jet printing. In this process, an image is formed by the ink-jet printing on a photosensitive plate which comprises a substrate and a photopolymerizable layer formed thereon. The image is capable of transmitting light to cure the photopolymerizable layer and made from an oxygen barrier resin material. Accordingly, when the photosensitive plate having the image is exposed to light to cure, oxygen inhibits polymerization reaction at the portion without the oxygen barrier film, but the portion with the oxygen barrier film permits the polymerization reaction to cure. The uncured portion is removed by development to create a lithographic printing plate.
The ink which is used for ink-jet printing is generally aqueous, but the surface of the photopolymerizable layer on which the ink is printed is hydrophobic. Accordingly, when the image is printed by ink-jet printing, some problems such as repelling or spreading of ink occur and do not provide a lithographic printing plate having good resolution. The surface of the photopolymerizable layer is generally tacky and therefore is difficult to handle. Further, the polymerizable monomers contained in the photopolymerizable layer volatilize a little and therefore reduce photosensitivity and curing degree, thus lowering printing durability.
Also, the surface energy of the photopolymerizable layer varies on the type of compositions and therefore the degree of repelling or spreading of ink varies according thereto. Thus, the resolution of the obtained printing plate also varies.